Neutron detector



March 31, 1959 L. MUSICANT NEUTRON DETECTOR Filed 061;. 25, 1954 AMPLlFlER POWER SUPPLY LOUIS MUSICNT INVENTOR ATTORNEY United hStaffa .Patent O NEUTRON DETECTOR Louis Musicant, Irvington, NJ., assignor to National adiac, Inc., Newark, NJ., a corporation of New ersey Application October 25, 1954, Serial No. 464,256

. i -8 Claims. (Cl. ZSO-71.5)

This invention relates to neutron detectors and has particular reference to polymers of boron mixed with fiuorescent substances used in conjunction with a light Sensitive element.

The detection of neutrons'has been accomplished in the past in various ways, such as, the employing of scintillation crystals of lithium iodide mixed with various fluorescent powders, Geiger counters having a boron compound on the inner surface of the glass envelope or iilled with boron trifluoride gas, and liquid scintllators. Each of these detectors has inherent disadvantages which prohibit their use except as laboratory instruments. The lithium iodide crystals are very expensive and are diificult to grow. Geiger tubes are fragile and with a boron compound within the envelope have a short Operating life. The liquid scintillators are unwieldly and have a low eiciency.

In order to overcome the 'above disadvantages a number of boron ester polymers have been prepared containing fluorescent substances which are cheap and easy to manufacture. They can be protected against mechanical shock and are efficient neutron detectors'. One such polymer is polyalcohol borate dispersed with powdered zinc sulfide. The boron content of this detector has been found to be between 23 and 24 percent by actual analysis and this high percentage of boron accounts for the relatively high efficiency. Other detectors such as polyethylene glycol borate will be described hereinafter.

One of the objects of this invention is to provide an improved neutron detector which avoids one or more of the dsadvantages and lmitations of prior art detectors.

Another object of the invention is to reduce the cost of neutron detectors.

Another object of the invention is to eliminate the dead time which is always present in Geiger counters.

Another object of the invention is to increase the portability and ruggedness of neutron counters so they may be taken into the field and transported without fear of failure.

One feature of the invention includes the mixture of a fluorescent powder and a boron ester polymer. This mixture is translucent and produces light flashes when bombarded with neutrons.

Another feature of the invention includes a light sensitive element mounted adjacent to the above described mixture 'and enclosed in a Chamber which permits neutrons to pass but which excludes light and other types of penetrating radiation.

For a better understanding of the present invention, together with other and further objects thereof, reference is made to the following description taken in connection with the accompanying drawing.

The single figure is a block diagram showing the container in section. A container 10 is provided for supporting a plastic block 11 and a photoelectric tube 12. The container walls may be made of lead or steel sheet in order to shield the plastic block from light and other forms of penetrating radiation such as alpha, beta,

gamma, or X-rays. If it is known that such latter radiation is absent the only shield required is a light shield.

A photoelectric cell 12 is indicated in the drawing because it has been found that such a device is more Sensitive than any otherl form of light Sensitive device.

However, any form of transducer which converts light' energy to electric energy may be used in this arrangement.

The electrical variations provided by the photocell 12 are transferred over conductors which pass lthrough lighttight insulators 13 and are connected tol an amplifier 14 if necessary. A power supply 15 is connected to the amplifier and also Supplies the photocell with Operating.

voltage. An indicator 16 is connectedto. the amplifier output and shows the extent of the light generated by the plastic block 11 and is therefore a Vmeasure of the number of neutrons which strike the block.

The plastic block 11 must 'have a high percentage of flashes from the portion farthest from the light Sensitivel element thereby producing the same effect as near by fiashes.

A neutron detector having the above listed characteristics can be made as follows: Forty grams of boric acid and ten grams of ethylene glycol are mixed together and heated to about 200 degrees centigrade until moisture is no longer evolved. The final product weighs 32.4 grams and contains 21.6 percent boron. By increasing the proportion of boric acid to ethylene glycol, the boron content can be increased to 24 percent. To the above liquid is now added 64.8 grams of -activated zinc sulfide in powdered form having a particle size within the range of five to ten microns. One such grade of zinc sulfide is known commercially as du Pont 2Bl-'ZnS. This mixture is stirred thoroughly to obtain uniform dispersion and the mixture is cooled to room temperature. 'Ihe resultant product may be stored in a desiccator to protect it from the moisture in the atmosphere or it may be processed immediately to form plates or blocks. The molding may =be done by placing the material in a prepared mold in a hydraulic press. The mold is heated to a temperature of about degrees centigrade and subjected to a pressure of 5,000 to 10,000 pounds per square inch. The result is a translucent block of polyethylene glycol borate having evenly dispersed zinc sulfide and more than 20 percent boron content.

Another formulation is produced as follows: Five grams trimethylol ethane and fifteen grams of boric acid are heated together gently in a test tube until all the moisture is driven out. The recovered plastic is polytrimethylol borate and weighs 14.6 grams. To this is added 29.2 grams of powdered zinc sulfide. The mixture is heated to about degrees centigrade and stirred to produce a homogeneous mass. The resulting mixture j can be molded in the same manner as the polyethylene Patnfea Mar. 31, 1959;

use illustratcd'. From the, above description it will be obvious that other similar'products can be made without departing from the field of the invention which should be limited only buy theSCQPQ of the appended claims.

I claim:

l. A neutron detectot comprising a transparent solid homogneous mixture, ofv a fiuorescent substance and a boron ester polyrner, said mixture containing at least 20 Prcntz of boron. byweight.

2. A neutron detector which produces light when bombarded by neutrons comprising a transparent solid homogeneous mixture` of a fluorescent substance and a boron ester polymer, said mixture containing at least 20 percent of boron by weight.

31. A neutron detectoras set forth in claim 2 wherein said fluorescent substance emts flashes. of light when bombarded' by 'alpha' particles.v

4. A neutron detector which produces light when bombard'ed by neutrons comprising a solid transparent homogeneous mxture of al fiuol'escentv substance and polyethylene glycol borate, said' mixture containing at least 20 percent of boron by Weight.

5. A neutron detector as set forth in claim 4 wherein said fluorescent substance is zinc sulfide.

6. A neutron detector as set forth in claim 2 wherein said boron ester polymer s polytrimethylol borate.

7. A neutron detector as set forth in claim 2 wherein said solid mixture is mounted adjacent to a light Sensitive element.

8. A neutron detector as set forth in claim 7 whereiny said light Sensitive element converts light energyV to electrical energy and includes conductive connections to an electrical indicating means.

References Cited in the file of this patent Two Liquid Scintillation Neutron Detectors, by Muehlhause and Thomas, from Nucleonics, vol. ll, No. 1, January 1953Z pp. 44,l 45..

A Fast Neutronl Scintillator, by- Emmerich, iromrthev Reviewof Scientific Instruments,` vol. 25, No. l, January 1954, pp. 69, 70. 

1. A NEUTRON DETECTOR COMPRISING A TRANSPARENT SOLID HOMOGENEOUS MIXTURE OF A FLUORESCENT SUBSTANCE AND A BORON ESTER POLYMER, SAID MIXTURE CONTAINING AT LEAST 20 PERCENT OF BORON BY WEIGHT. 